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			<h1>MySQL 查询数据</h1>
<p>MySQL 数据库使用SQL SELECT语句来查询数据。</p>
<p>
你可以通过 mysql&gt; 命令提示窗口中在数据库中查询数据，或者通过PHP脚本来查询数据。
</p>

<h3>语法</h3>
<p>
以下为在MySQL数据库中查询数据通用的 SELECT 语法：
</p>
<pre>
SELECT column_name,column_name
FROM table_name
[WHERE Clause]
[LIMIT N][ OFFSET M]
</pre>
<ul>
	<li>查询语句中你可以使用一个或者多个表，表之间使用逗号(,)分割，并使用WHERE语句来设定查询条件。</li>
	<li>SELECT 命令可以读取一条或者多条记录。</li>
	<li>你可以使用星号（*）来代替其他字段，SELECT语句会返回表的所有字段数据</li>
	<li>你可以使用 WHERE 语句来包含任何条件。</li>
<li>你可以使用 LIMIT 属性来设定返回的记录数。</li>
	<li>你可以通过OFFSET指定SELECT语句开始查询的数据偏移量。默认情况下偏移量为0。</li>
	
</ul>
<hr>
<h2>通过命令提示符获取数据</h2>
<p>以下实例我们将通过 SQL SELECT 命令来获取 MySQL 数据表 runoob_tbl 的数据：
</p>
<h3>实例</h3>
<p>以下实例将返回数据表 runoob_tbl 的所有记录:</p>
<div class="example">
<h2 class="example">读取数据表：</h2>
<div class="example_code">
<div class="hl-main"><span class="hl-reserved">select</span><span class="hl-code"> * </span><span class="hl-reserved">from</span><span class="hl-code"> </span><span class="hl-identifier">runoob_tbl</span><span class="hl-code">;</span></div>
</div>
<p>输出结果：</p>
<div class="example_code">
<img decoding="async" src="//www.runoob.com/wp-content/uploads/2014/03/DB742246-84F3-4447-BD43-6BAEADD7CA91.jpg"></p>
</div>
</div>
<hr>
<h2>使用PHP脚本来获取数据</h2>
<p>
使用 PHP 函数的 <span class="marked">mysqli_query()</span> 及 <span class="marked">SQL SELECT</span> 命令来获取数据。</p><p>
该函数用于执行 SQL 命令，然后通过 PHP 函数 <span class="marked">mysqli_fetch_array() </span>
来使用或输出所有查询的数据。</p>
<p>
<span class="marked">mysqli_fetch_array() </span> 函数从结果集中取得一行作为关联数组，或数字数组，或二者兼有
返回根据从结果集取得的行生成的数组，如果没有更多行则返回 false。
</p><p>
以下实例为从数据表 runoob_tbl 中读取所有记录。</p>
<h3>实例</h3>
<p>尝试以下实例来显示数据表 runoob_tbl 的所有记录。
<div class="example"> 
<h2 class="example">使用 mysqli_fetch_array MYSQLI_ASSOC 参数获取数据：</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-inlinetags">&lt;?php</span><span class="hl-code">
</span><span class="hl-var">$dbhost</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">localhost</span><span class="hl-quotes">'</span><span class="hl-code">;  </span><span class="hl-comment">//</span><span class="hl-comment"> mysql服务器主机地址</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbuser</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">root</span><span class="hl-quotes">'</span><span class="hl-code">;            </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbpass</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">123456</span><span class="hl-quotes">'</span><span class="hl-code">;          </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名密码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$conn</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_connect</span><span class="hl-brackets">(</span><span class="hl-var">$dbhost</span><span class="hl-code">, </span><span class="hl-var">$dbuser</span><span class="hl-code">, </span><span class="hl-var">$dbpass</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$conn</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">连接失败: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-comment">//</span><span class="hl-comment"> 设置编码，防止中文乱码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-code"> , </span><span class="hl-quotes">&quot;</span><span class="hl-string">set names utf8</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
 
</span><span class="hl-var">$sql</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">SELECT runoob_id, runoob_title, 
        runoob_author, submission_date
        FROM runoob_tbl</span><span class="hl-quotes">'</span><span class="hl-code">;
 
</span><span class="hl-identifier">mysqli_select_db</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">RUNOOB</span><span class="hl-quotes">'</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-var">$retval</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-var">$sql</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$retval</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">无法读取数据: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;h2&gt;菜鸟教程 mysqli_fetch_array 测试&lt;/h2&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;table border=&quot;1&quot;&gt;&lt;tr&gt;&lt;td&gt;教程 ID&lt;/td&gt;&lt;td&gt;标题&lt;/td&gt;&lt;td&gt;作者&lt;/td&gt;&lt;td&gt;提交日期&lt;/td&gt;&lt;/tr&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">while</span><span class="hl-brackets">(</span><span class="hl-var">$row</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_fetch_array</span><span class="hl-brackets">(</span><span class="hl-var">$retval</span><span class="hl-code">, </span><span class="hl-identifier">MYSQLI_ASSOC</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;tr&gt;&lt;td&gt; </span><span class="hl-var">{$row['runoob_id']}</span><span class="hl-string">&lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row['runoob_title']}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row['runoob_author']}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row['submission_date']}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;/tr&gt;</span><span class="hl-quotes">&quot;</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;/table&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-identifier">mysqli_close</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-inlinetags">?&gt;</span></div>
</div>
<p>输出结果如下所示：</p>
<div class="example_code">
<img decoding="async" src="//www.runoob.com/wp-content/uploads/2014/03/9B08EFFD-6326-4C2B-BFCA-171714FC018D.jpg">
</div>

</div>
<p>以上实例中，读取的每行记录赋值给变量 $row，然后再打印出每个值。</p>
<p><strong>注意：</strong>记住如果你需要在字符串中使用变量，请将变量置于花括号。</p>
<p>
在上面的例子中，PHP mysqli_fetch_array() 函数第二个参数为 <strong>MYSQLI_ASSOC</strong>，

设置该参数查询结果返回关联数组，你可以使用字段名称来作为数组的索引。</p>
<p>
PHP 提供了另外一个函数 <strong>mysqli_fetch_assoc()</strong>, 该函数从结果集中取得一行作为关联数组。
返回根据从结果集取得的行生成的关联数组，如果没有更多行，则返回 false。
</p>
<h3>实例</h3>
<p>
尝试以下实例，该实例使用了 <strong>mysqli_fetch_assoc()</strong> 函数来输出数据表 runoob_tbl 的所有记录：</p>
<div class="example"> 
<h2 class="example">使用 mysqli_fetch_assoc 获取数据：</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-inlinetags">&lt;?php</span><span class="hl-code">
</span><span class="hl-var">$dbhost</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">localhost:3306</span><span class="hl-quotes">'</span><span class="hl-code">;  </span><span class="hl-comment">//</span><span class="hl-comment"> mysql服务器主机地址</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbuser</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">root</span><span class="hl-quotes">'</span><span class="hl-code">;            </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbpass</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">123456</span><span class="hl-quotes">'</span><span class="hl-code">;          </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名密码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$conn</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_connect</span><span class="hl-brackets">(</span><span class="hl-var">$dbhost</span><span class="hl-code">, </span><span class="hl-var">$dbuser</span><span class="hl-code">, </span><span class="hl-var">$dbpass</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$conn</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">连接失败: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-comment">//</span><span class="hl-comment"> 设置编码，防止中文乱码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-code"> , </span><span class="hl-quotes">&quot;</span><span class="hl-string">set names utf8</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
 
</span><span class="hl-var">$sql</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">SELECT runoob_id, runoob_title, 
        runoob_author, submission_date
        FROM runoob_tbl</span><span class="hl-quotes">'</span><span class="hl-code">;
 
</span><span class="hl-identifier">mysqli_select_db</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">RUNOOB</span><span class="hl-quotes">'</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-var">$retval</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-var">$sql</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$retval</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">无法读取数据: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;h2&gt;菜鸟教程 mysqli_fetch_assoc 测试&lt;/h2&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;table border=&quot;1&quot;&gt;&lt;tr&gt;&lt;td&gt;教程 ID&lt;/td&gt;&lt;td&gt;标题&lt;/td&gt;&lt;td&gt;作者&lt;/td&gt;&lt;td&gt;提交日期&lt;/td&gt;&lt;/tr&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">while</span><span class="hl-brackets">(</span><span class="hl-var">$row</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_fetch_assoc</span><span class="hl-brackets">(</span><span class="hl-var">$retval</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;tr&gt;&lt;td&gt; </span><span class="hl-var">{$row['runoob_id']}</span><span class="hl-string">&lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row['runoob_title']}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row['runoob_author']}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row['submission_date']}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;/tr&gt;</span><span class="hl-quotes">&quot;</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;/table&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-identifier">mysqli_close</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-inlinetags">?&gt;</span></div>
</div>
<p>输出结果如下所示：</p>
<div class="example_code">
<img decoding="async" src="//www.runoob.com/wp-content/uploads/2014/03/9B7E791A-6461-4B83-B89D-11D592C07AC5.jpg">
</div>
</div>
<p>
你也可以使用常量 MYSQLI_NUM 作为 PHP mysqli_fetch_array() 函数的第二个参数，返回数字数组。
</p>
<h3>实例</h3>
<p>以下实例使用 <strong> MYSQLI_NUM </strong> 参数显示数据表 runoob_tbl 的所有记录:
</p>
<div class="example"> 
<h2 class="example">使用 mysqli_fetch_array MYSQLI_NUM 参数获取数据：</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-inlinetags">&lt;?php</span><span class="hl-code">
</span><span class="hl-var">$dbhost</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">localhost:3306</span><span class="hl-quotes">'</span><span class="hl-code">;  </span><span class="hl-comment">//</span><span class="hl-comment"> mysql服务器主机地址</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbuser</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">root</span><span class="hl-quotes">'</span><span class="hl-code">;            </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbpass</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">123456</span><span class="hl-quotes">'</span><span class="hl-code">;          </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名密码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$conn</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_connect</span><span class="hl-brackets">(</span><span class="hl-var">$dbhost</span><span class="hl-code">, </span><span class="hl-var">$dbuser</span><span class="hl-code">, </span><span class="hl-var">$dbpass</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$conn</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">连接失败: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-comment">//</span><span class="hl-comment"> 设置编码，防止中文乱码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-code"> , </span><span class="hl-quotes">&quot;</span><span class="hl-string">set names utf8</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
 
</span><span class="hl-var">$sql</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">SELECT runoob_id, runoob_title, 
        runoob_author, submission_date
        FROM runoob_tbl</span><span class="hl-quotes">'</span><span class="hl-code">;
 
</span><span class="hl-identifier">mysqli_select_db</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">RUNOOB</span><span class="hl-quotes">'</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-var">$retval</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-var">$sql</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$retval</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">无法读取数据: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;h2&gt;菜鸟教程 mysqli_fetch_array 测试&lt;/h2&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;table border=&quot;1&quot;&gt;&lt;tr&gt;&lt;td&gt;教程 ID&lt;/td&gt;&lt;td&gt;标题&lt;/td&gt;&lt;td&gt;作者&lt;/td&gt;&lt;td&gt;提交日期&lt;/td&gt;&lt;/tr&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">while</span><span class="hl-brackets">(</span><span class="hl-var">$row</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_fetch_array</span><span class="hl-brackets">(</span><span class="hl-var">$retval</span><span class="hl-code">, </span><span class="hl-identifier">MYSQLI_NUM</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;tr&gt;&lt;td&gt; </span><span class="hl-var">{$row[0]}</span><span class="hl-string">&lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row[1]}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row[2]}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row[3]}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;/tr&gt;</span><span class="hl-quotes">&quot;</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;/table&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-identifier">mysqli_close</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-inlinetags">?&gt;</span></div>
</div>
<p>输出结果如下所示：</p>
<div class="example_code">
<img decoding="async" src="//www.runoob.com/wp-content/uploads/2014/03/9B08EFFD-6326-4C2B-BFCA-171714FC018D.jpg">
</div>

</div>
<p>以上三个实例输出结果都一样。</p>

<hr>
<h2>
内存释放</h2>
<p>在我们执行完 SELECT 语句后，释放游标内存是一个很好的习惯。
</p><p>
可以通过 PHP 函数 mysqli_free_result() 来实现内存的释放。
</p>
<p>
以下实例演示了该函数的使用方法。
</p>
<h3>实例</h3>
<p>尝试以下实例:</p>
<div class="example"> 
<h2 class="example">使用 mysqli_free_result 释放内存：</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-inlinetags">&lt;?php</span><span class="hl-code">
</span><span class="hl-var">$dbhost</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">localhost:3306</span><span class="hl-quotes">'</span><span class="hl-code">;  </span><span class="hl-comment">//</span><span class="hl-comment"> mysql服务器主机地址</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbuser</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">root</span><span class="hl-quotes">'</span><span class="hl-code">;            </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$dbpass</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">123456</span><span class="hl-quotes">'</span><span class="hl-code">;          </span><span class="hl-comment">//</span><span class="hl-comment"> mysql用户名密码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-var">$conn</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_connect</span><span class="hl-brackets">(</span><span class="hl-var">$dbhost</span><span class="hl-code">, </span><span class="hl-var">$dbuser</span><span class="hl-code">, </span><span class="hl-var">$dbpass</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$conn</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">连接失败: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-comment">//</span><span class="hl-comment"> 设置编码，防止中文乱码</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-code"> , </span><span class="hl-quotes">&quot;</span><span class="hl-string">set names utf8</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;
 
</span><span class="hl-var">$sql</span><span class="hl-code"> = </span><span class="hl-quotes">'</span><span class="hl-string">SELECT runoob_id, runoob_title, 
        runoob_author, submission_date
        FROM runoob_tbl</span><span class="hl-quotes">'</span><span class="hl-code">;
 
</span><span class="hl-identifier">mysqli_select_db</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">RUNOOB</span><span class="hl-quotes">'</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-var">$retval</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_query</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-var">$conn</span><span class="hl-code">, </span><span class="hl-var">$sql</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-reserved">if</span><span class="hl-brackets">(</span><span class="hl-code">! </span><span class="hl-var">$retval</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">die</span><span class="hl-brackets">(</span><span class="hl-quotes">'</span><span class="hl-string">无法读取数据: </span><span class="hl-quotes">'</span><span class="hl-code"> . </span><span class="hl-identifier">mysqli_error</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;h2&gt;菜鸟教程 mysqli_fetch_array 测试&lt;/h2&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;table border=&quot;1&quot;&gt;&lt;tr&gt;&lt;td&gt;教程 ID&lt;/td&gt;&lt;td&gt;标题&lt;/td&gt;&lt;td&gt;作者&lt;/td&gt;&lt;td&gt;提交日期&lt;/td&gt;&lt;/tr&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-reserved">while</span><span class="hl-brackets">(</span><span class="hl-var">$row</span><span class="hl-code"> = </span><span class="hl-identifier">mysqli_fetch_array</span><span class="hl-brackets">(</span><span class="hl-var">$retval</span><span class="hl-code">, </span><span class="hl-identifier">MYSQLI_NUM</span><span class="hl-brackets">)</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;tr&gt;&lt;td&gt; </span><span class="hl-var">{$row[0]}</span><span class="hl-string">&lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row[1]}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row[2]}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;td&gt;</span><span class="hl-var">{$row[3]}</span><span class="hl-string"> &lt;/td&gt; </span><span class="hl-quotes">&quot;</span><span class="hl-code">.
         </span><span class="hl-quotes">&quot;</span><span class="hl-string">&lt;/tr&gt;</span><span class="hl-quotes">&quot;</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-reserved">echo</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">&lt;/table&gt;</span><span class="hl-quotes">'</span><span class="hl-code">;
</span><span class="hl-comment">//</span><span class="hl-comment"> 释放内存</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">mysqli_free_result</span><span class="hl-brackets">(</span><span class="hl-var">$retval</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-identifier">mysqli_close</span><span class="hl-brackets">(</span><span class="hl-var">$conn</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-inlinetags">?&gt;</span></div>
</div>
<p>输出结果如下所示：</p>
<div class="example_code">
<img decoding="async" src="//www.runoob.com/wp-content/uploads/2014/03/9B08EFFD-6326-4C2B-BFCA-171714FC018D.jpg">
</div>

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/* CSS Triangles - see Trevor's post */
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    <span class="mw-headline" id="qa_headline">7  篇笔记</span>
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<div id="postcomments"  style="display:none;" >
	<ol class="commentlist">
		<li class="comment even thread-even depth-1" id="comment-28032"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;zhaojjiang</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;127***1900@qq.com</p></div><div id="runoobvote-id-28032" data-commid = "28032" class="upvotejs"><a class="upvote"></a> <span class="count">74</span></div></div><div class="comt-main" id="div-comment-28032"><pre>select _column,_column from _table [where Clause] [limit N][offset M]</pre>
<ul data-title="MYSQL LIMIT 用法" data-commid="29340"><li>
<span class="marked">select *</span> : 返回所有记录
</li><li>
<span class="marked">limit N</span> : 返回 N 条记录
</li><li>
<span class="marked">offset M</span> : 跳过 M 条记录, 默认 M=0, 单独使用似乎不起作用
</li><li>
<span class="marked">limit N,M</span> :  相当于 <strong>limit M offset N</strong> , 从第 N 条记录开始, 返回 M 条记录
</li></ul>
<p>实现分页：</p>

<pre>select * from _table limit (page_number-1)*lines_perpage, lines_perpage

或

select * from _table limit lines_perpage offset (page_number-1)*lines_perpage</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/28032">zhaojjiang</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;zhaojjiang</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;127***1900@qq.com</p></div></span>5年前 (2018-06-02)</div></div></li><!-- #comment-## -->
<li class="comment byuser comment-author-tianqixin bypostauthor odd alt thread-odd thread-alt depth-1" id="comment-29340"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;小太阳</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;cho***anyang@asfenqi.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://www.cnblogs.com/cai170221/p/7122289.html">&nbsp;&nbsp;参考地址</a></p></div><div id="runoobvote-id-29340" data-commid = "29340" class="upvotejs"><a class="upvote"></a> <span class="count">42</span></div></div><div class="comt-main" id="div-comment-29340"><p>MySQL limit 应用的一些例子。</p>
<p>语法格式：</p>
<pre>SELECT * FROM table LIMIT [offset,] rows | rows OFFSET offset</pre>
<p><strong>解析：</strong>LIMIT 子句可以被用于强制 SELECT 语句返回指定的记录数。LIMIT 接受一个或两个数字参数。参数必须是一个整数常量。如果给定两个参数，第一个参数指定第一个返回记录行的偏移量，第二个参数指定返回记录行的最大数目。初始记录行的偏移量是 0(而不是 1)： 为了与 PostgreSQL 兼容，MySQL 也支持句法： LIMIT # OFFSET #。</p>

<pre>mysql&gt; SELECT * FROM table LIMIT 5,10; // 检索记录行 6-15   
  
//如果只给定一个参数，它表示返回最大的记录行数目：    
mysql&gt; SELECT * FROM table LIMIT 5; //检索前 5 个记录行   
  
//换句话说，LIMIT n 等价于 LIMIT 0,n。</pre>

<h2>Mysql 的分页查询语句的性能分析</h2>

<p>MySql 分页 sql 语句，如果和 MSSQL 的 TOP 语法相比，那么 MySQL 的 LIMIT 语法要显得优雅了许多。使用它来分页是再自然不过的事情了。</p>
<p>2.1 最基本的分页方式：</p>
<pre>SELECT ... FROM ... WHERE ... ORDER BY ... LIMIT ...</pre>
  

<p>在中小数据量的情况下，这样的 SQL 足够用了，唯一需要注意的问题就是确保使用了索引。</p>

<p>举例来说，如果实际 SQL 类似下面语句，那么在 category_id, id 两列上建立复合索引比较好。</p>
<pre>SELECT * FROM articles WHERE category_id = 123 ORDER BY id LIMIT 50, 10</pre>

<p><strong>2.2 子查询的分页方式</strong></p>

 

<p>随着数据量的增加，页数会越来越多，查看后几页的 SQL 就可能类似：</p>
<pre>SELECT * FROM articles WHERE category_id = 123 ORDER BY id LIMIT 10000, 10</pre>

<p>一言以蔽之，就是越往后分页，LIMIT 语句的偏移量就会越大，速度也会明显变慢。</p>

<p>此时，我们可以通过子查询的方式来提高分页效率，大致如下：</p>
<pre>SELECT * FROM articles WHERE  id &gt;=
 (SELECT id FROM articles  WHERE category_id = 123 ORDER BY id LIMIT 10000, 1) LIMIT 10</pre>
<p><strong>2.3 JOIN 分页方式</strong></p>
<pre>SELECT * FROM `content` AS t1    
JOIN (SELECT id FROM `content` ORDER BY id desc LIMIT ".($page-1)*$pagesize.", 1) AS t2    
WHERE t1.id &lt;= t2.id ORDER BY t1.id desc LIMIT $pagesize;</pre>

<p>经过我的测试，join 分页和子查询分页的效率基本在一个等级上，消耗的时间也基本一致。</p>

<p>explain SQL语句：</p>
<pre>
id select_type table type possible_keys key key_len ref rows Extra

1 PRIMARY &lt;derived2&gt; system NULL NULL NULL NULL 1  

1 PRIMARY t1 range PRIMARY PRIMARY 4 NULL 6264 Using where

2 DERIVED content index NULL PRIMARY 4 NULL 27085 Using index
</pre>
 



 

<p><strong>为什么会这样呢？</strong>因为子查询是在索引上完成的，而普通的查询时在数据文件上完成的，通常来说，索引文件要比数据文件小得多，所以操作起来也会更有效率。</p>

 

<p>实际可以利用类似策略模式的方式去处理分页，比如判断如果是一百页以内，就使用最基本的分页方式，大于一百页，则使用子查询的分页方式。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">小太阳</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;小太阳</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;cho***anyang@asfenqi.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://www.cnblogs.com/cai170221/p/7122289.html">&nbsp;&nbsp;参考地址</a></p></div></span>5年前 (2018-07-09)</div></div></li><!-- #comment-## -->
<li class="comment byuser comment-author-tianqixin bypostauthor even thread-even depth-1" id="comment-29370"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;吴广东</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;128***9441@qq.com</p></div><div id="runoobvote-id-29370" data-commid = "29370" class="upvotejs"><a class="upvote"></a> <span class="count">91</span></div></div><div class="comt-main" id="div-comment-29370"><p>Mysql 简单查询语句，可以通过不同的查询语句进行套用。</p>

<pre>/*websites  表名   NAME alexa url country  字段*/
SELECT * FROM websites;                      /* 查询表所有数据 */

SELECT NAME FROM websites;                   /* 查询表字段数据 */

SELECT * FROM websites where name = "广西";   /* 查询表字段下条件数据 */

SELECT * from websites where name like "_o%"; /* 模糊查询表下数据 */

SELECT * FROM websites where id BETWEEN "1" AND "5";    /* 查询表下字段范围数据 */

SELECT * FROM websites WHERE name in ("广西","百度");    /* 查询表字段下固定条件数据 */

SELECT DISTINCT country FROM Websites;                  /* 查询去重值 */

SELECT * FROM Websites WHERE country = "CN" AND alexa &gt; 50;  /*查询表下范围条件数据*/

SELECT * FROM Websites WHERE country = "USA" OR country="sh"; /* 查询表下条件不同值 */

SELECT * FROM Websites ORDER BY alexa;                      /* 查询表下值排序结果 */

SELECT * FROM Websites ORDER BY alexa DESC;                 /* 查询表下排序结果降序 */

SELECT * FROM Websites LIMIT 2;      /* 查询表下范围数据 */

SELECT name as zzz from websites;    /*别名查询表下数据*/
</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">吴广东</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;吴广东</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;128***9441@qq.com</p></div></span>5年前 (2018-07-10)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-34979"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Narule</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;jun***r33@sina.com</p></div><div id="runoobvote-id-34979" data-commid = "34979" class="upvotejs"><a class="upvote"></a> <span class="count">39</span></div></div><div class="comt-main" id="div-comment-34979"><p><strong>sql 语句的关联查询 </strong></p>

<p>左关联: <span class="marked">left join ... on ...</span>  </p>

<p>右关联: <span class="marked">right join... on ...</span></p>

<p>格式:</p>
 <pre>select 字段 from 表1 left join 表2 on  条件 (一般为表1与表2的关联条件)</pre>

<p>查询用户的所有订单信息 :</p>
<pre>user 用户表   
orders 订单表</pre>

<pre>select * from user left join orders on user.id = orders.user_id </pre>
<p>稍微复杂点    统计用户的订单数量 (需要分组,通过用户的id)</p>

<pre>select user.username,orders.id,count(*) from user right join orders on user.id = orders.user_id GROUP BY user.id;
mysql&gt; select user.username,orders.id,count(*) from user right join orders on user.id = orders.user_id GROUP BY user.id;
+----------+----+----------+
| username | id | count(*) |
+----------+----+----------+
| 王五     |  3 |        2 |
| 张三     |  5 |        1 |
+----------+----+----------+
2 rows in set (0.07 sec)</pre>
<p>这里显示名为王五(id=3)的用户有2个订单 张三(id=5)1个订单 </p>

<p>这里是右关联查询,用右关联查询是有道理的, 因为左关联和有关联 是有差别的查询,区别:left  join on 左边的表为主表 right join on 右边的表为主表   </p>

<p>这个统计订单的查询有一个问题  就是 用户表中有用户新信息,但是这个用户没有订单信息</p>

<p>请看下面的查询;</p>

<pre>---------------------
单表查询
---------------------
mysql&gt; select * from user;
+----+----------+------------+------+----------+
| id | username | birthday   | sex  | address  |
+----+----------+------------+------+----------+
|  1 | 王五     | 2017-11-25 | 3    | 南阳     |
| 10 | 张三     | 2014-07-10 | 1    | 北京市   |
| 16 | 张小明   | NULL       | 1    | 河南郑州 |
| 22 | 陈小明   | NULL       | 1    | 河南郑州 |
| 24 | 张三丰   | NULL       | 1    | 河南郑州 |
| 25 | 陈小明   | NULL       | 1    | 河南郑州 |
| 26 | 王五     | NULL       | NULL | NULL     |
| 29 | 小黑     | 2017-11-26 | NULL | NULL     |
| 30 | 抖森     | 2017-11-25 | 1    | 山村     |
+----+----------+------------+------+----------+
9 rows in set (0.03 sec)

mysql&gt; select * from orders;
+----+---------+---------+---------------------+------+
| id | user_id | number  | createtime          | note |
+----+---------+---------+---------------------+------+
|  3 |       1 | 1000010 | 2015-02-04 13:22:35 | NULL |
|  4 |       1 | 1000011 | 2015-02-03 13:22:41 | NULL |
|  5 |      10 | 1000012 | 2015-02-12 16:13:23 | NULL |
+----+---------+---------+---------------------+------+
3 rows in set (0.03 sec)
--------------------------
关联查询
-------------------------
左关联
------------------------
mysql&gt; select user.* ,orders.number from user  left join orders on user.id = orders.user_id;
+----+----------+------------+------+----------+---------+
| id | username | birthday   | sex  | address  | number  |
+----+----------+------------+------+----------+---------+
|  1 | 王五     | 2017-11-25 | 3    | 南阳     | 1000010 |
|  1 | 王五     | 2017-11-25 | 3    | 南阳     | 1000011 |
| 10 | 张三     | 2014-07-10 | 1    | 北京市   | 1000012 |
| 16 | 张小明   | NULL       | 1    | 河南郑州 | NULL    |
| 22 | 陈小明   | NULL       | 1    | 河南郑州 | NULL    |
| 24 | 张三丰   | NULL       | 1    | 河南郑州 | NULL    |
| 25 | 陈小明   | NULL       | 1    | 河南郑州 | NULL    |
| 26 | 王五     | NULL       | NULL | NULL     | NULL    |
| 29 | 小黑     | 2017-11-26 | NULL | NULL     | NULL    |
| 30 | 抖森     | 2017-11-25 | 1    | 山村     | NULL    |
+----+----------+------------+------+----------+---------+
10 rows in set (0.04 sec)
-----------------------------
右关联
-----------------------------
mysql&gt; select user.* ,orders.number from user right join orders on user.id = orders.user_id;
+----+----------+------------+-----+---------+---------+
| id | username | birthday   | sex | address | number  |
+----+----------+------------+-----+---------+---------+
|  1 | 王五     | 2017-11-25 | 3   | 南阳    | 1000010 |
|  1 | 王五     | 2017-11-25 | 3   | 南阳    | 1000011 |
| 10 | 张三     | 2014-07-10 | 1   | 北京市  | 1000012 |
+----+----------+------------+-----+---------+---------+
3 rows in set (0.05 sec)</pre>
<p>很明显此处错误的选择left会导致查出不必要的数据,可以说是垃圾信息,因为是要查出订单信息(携带用户信息)没有订单的用户就不必要查询出来。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">Narule</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Narule</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;jun***r33@sina.com</p></div></span>5年前 (2018-09-11)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-34980"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Narule</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;jun***r33@sina.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://www.cnblogs.com/geaozhang/p/6745147.html">&nbsp;&nbsp;参考地址</a></p></div><div id="runoobvote-id-34980" data-commid = "34980" class="upvotejs"><a class="upvote"></a> <span class="count">11</span></div></div><div class="comt-main" id="div-comment-34980"><p><strong>多级关联查询以及聚合函数的使用</strong></p>

<p>没事多尝试下 最多报错, 不会损坏数据库(最多也是测试环境数据库,难道你还是在生产环境学习sql?)</p>

<p>多级关联和两个表关联查询没什么区别 大胆的去尝试就知道了,直接在后面加 <span class="marked">left jion on</span></p>

<pre>mysql&gt; select * from user right join orders on user.id = orders.user_id right join orderdetail on orders.id = orderdetail.orders_id left join items on items.id = orderdetail.items_id;
+----+----------+------------+-----+---------+----+---------+---------+---------------------+------+----+-----------+----------+-----------+----+---------+--------+--------+------------------------------------------+---------------------+
| id | username | birthday   | sex | address | id | user_id | number  | createtime          | note | id | orders_id | items_id | items_num | id | name    | price  | detail | pic                                      | createtime          |
+----+----------+------------+-----+---------+----+---------+---------+---------------------+------+----+-----------+----------+-----------+----+---------+--------+--------+------------------------------------------+---------------------+
|  1 | 王五     | 2017-11-25 | 3   | 南阳    |  3 |       1 | 1000010 | 2015-02-04 13:22:35 | NULL |  1 |         3 |        1 |         1 |  1 | 台式机1 | 3000.0 |        | e2269631-2190-45ed-a215-b8a8dc0dc3d7.png | 2015-02-03 13:22:50 |
|  1 | 王五     | 2017-11-25 | 3   | 南阳    |  3 |       1 | 1000010 | 2015-02-04 13:22:35 | NULL |  2 |         3 |        2 |         3 |  2 | 笔记本  | 6000.0 |        | fffb17d9-0280-461a-b9db-9927ffe5c1b7.png | 2015-02-09 13:22:57 |
|  1 | 王五     | 2017-11-25 | 3   | 南阳    |  4 |       1 | 1000011 | 2015-02-03 13:22:41 | NULL |  3 |         4 |        3 |         4 |  3 | 背包    |  200.0 |        | NULL                                     | 2015-02-06 13:23:02 |
|  1 | 王五     | 2017-11-25 | 3   | 南阳    |  4 |       1 | 1000011 | 2015-02-03 13:22:41 | NULL |  4 |         4 |        2 |         3 |  2 | 笔记本  | 6000.0 |        | fffb17d9-0280-461a-b9db-9927ffe5c1b7.png | 2015-02-09 13:22:57 |
+----+----------+------------+-----+---------+----+---------+---------+---------------------+------+----+-----------+----------+-----------+----+---------+--------+--------+------------------------------------------+---------------------+
4 rows in set (0.08 sec)</pre>
<p><strong>复杂的sql </strong></p>

<p>要求: 统计用户的各订单价格</p>

<p>(注意,这里每个订单中可能有几个订单项,比如一个订单里面有书和鞋,这里面的订单价格就是 书的价格+鞋的价格)</p>

<pre>mysql&gt; select user.id,user.username,user.address,orders.number,sum(items.price) from user right join orders on user.id = orders.user_id right join orderdetail on orders.id = orderdetail.orders_id left join items on items.id = orderdetail.items_id GROUP BY orders.number;
+----+----------+---------+---------+------------------+
| id | username | address | number  | sum(items.price) |
+----+----------+---------+---------+------------------+
|  1 | 王五     | 南阳    | 1000010 |           9000.0 |
|  1 | 王五     | 南阳    | 1000011 |           6200.0 |
+----+----------+---------+---------+------------------+
2 rows in set (0.11 sec)


mysql&gt; 
select user.id,user.username,user.address,orders.number,items.name, items.price from user right join orders on user.id = orders.user_id right join orderdetail on orders.id = orderdetail.orders_id left join items on items.id = orderdetail.items_id;
+----+----------+---------+---------+---------+--------+
| id | username | address | number  | name    | price  |
+----+----------+---------+---------+---------+--------+
|  1 | 王五     | 南阳    | 1000010 | 台式机1 | 3000.0 |
|  1 | 王五     | 南阳    | 1000010 | 笔记本  | 6000.0 |
|  1 | 王五     | 南阳    | 1000011 | 背包    |  200.0 |
|  1 | 王五     | 南阳    | 1000011 | 笔记本  | 6000.0 |
+----+----------+---------+---------+---------+--------+
4 rows in set (0.08 sec)</pre>
<p>第一个查询统计价格用到了 sum() 函数 用来累加求和 属于聚合函数</p>

<p>数据聚合函数的还有 avg()求平均值 max()最大值 min()最小值 count()统计数据条数。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">Narule</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Narule</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;jun***r33@sina.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://www.cnblogs.com/geaozhang/p/6745147.html">&nbsp;&nbsp;参考地址</a></p></div></span>5年前 (2018-09-11)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-41755"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;浪迹</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;248***4514@qq.com</p></div><div id="runoobvote-id-41755" data-commid = "41755" class="upvotejs"><a class="upvote"></a> <span class="count">12</span></div></div><div class="comt-main" id="div-comment-41755"><p>数据库查询数据还可以选择查询哪一部分的数据。</p>

<p>SQL 语句：</p>
<pre>select*from  表名  order by 字段  （倒序/升序） limit   start,count;</pre>

<p>查询数据库中学生表逆序的 5 条数据：</p>
<pre>select*from student order by id desc limit 0,5;</pre>


<ul><li>
order by id: 通过id来查询</li><li>
desc: 表示倒序，可替换成 asc ，表示升序</li><li>
start: 开始（升序第一条是0，降序最后一条是0）</li><li>
count: 查询的个数</li></ul>

<pre>mysql&gt; select*from student;
+----+--------+------+
| id | name   | age  |
+----+--------+------+
| 16 | gege   |   50 |
| 18 | didi   |   23 |
| 19 | gege   |   23 |
| 20 | yeye   |   23 |
| 21 | jiujiu |  101 |
| 25 | gege   |   13 |
| 29 | kaka   |    2 |
| 32 | haha   |   20 |
| 33 | ghg    |   20 |
| 34 | dijia  |   20 |
| 38 | daina  |   20 |
+----+--------+------+
11 rows in set (0.00 sec)
mysql&gt; select*from student order by id desc limit 0,5;

+----+-------+------+
| id | name  | age  |
+----+-------+------+
| 38 | daina |   20 |
| 34 | dijia |   20 |
| 33 | ghg   |   20 |
| 32 | haha  |   20 |
| 29 | kaka  |    2 |
+----+-------+------+
5 rows in set (0.00 sec)</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">浪迹</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;浪迹</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;248***4514@qq.com</p></div></span>4年前 (2019-04-11)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-44574"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;抹茶</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;173***2517@qq.com</p></div><div id="runoobvote-id-44574" data-commid = "44574" class="upvotejs"><a class="upvote"></a> <span class="count">26</span></div></div><div class="comt-main" id="div-comment-44574"><p><strong>MySQL 分页查询</strong></p>

<p>后端在写 mysql 语句来处理前端的分页查询请求其实非常简单，用 limit 就行。</p>

<p>例：</p>

<pre>select * from foo limit 100,200</pre>

<p>100 是指偏移，200 是指查询条数。</p>

<p>所以后端代码需要接收两个参数：偏移（offset），查询条数（rows）。</p>
<p>前端传递给后端的参数可以是page_num(第几页)，page_size（每一页显示多少条数据），后端接收到 page_num 和 page_size 之后可以通过 (page_num - 1) * page_size 表达式换算 offset，查询条数(rows)就是 page_size</p>

<p>整个后端代码：</p>

<pre>def paging(page_count, page_size):
    table_name = 'Test'
    offset = (page_count - 1) * page_size
    sql = 'select * from %s limit %d, %d'%(table_name, offset, page_size)
    print(sql)
    conn.execute(sql)
    conn.commit()</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">抹茶</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;抹茶</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;173***2517@qq.com</p></div></span>4年前 (2019-07-13)</div></div></li><!-- #comment-## -->
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